User equipment (UE), also known as mobile stations, wireless terminals and/or mobile terminals are enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two user equipment, between a user equipment and a wire connected telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The user equipment may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The user equipments in the present text may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The radio network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the radio network node/base station at a base station site. One base station, situated on the base station site, may serve one or several cells. The radio network nodes communicate over the air interface operating on radio frequencies with the user equipment units within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunication System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communication.
UMTS is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as high data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of an UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system.
Handover procedure in LTE, similar to other wireless communication system, is designed to guarantee the continuous coverage upon UE's movement. In an E-UTRAN, a UE 130 is wirelessly connected to a serving network node 110 and a neighbour network node 120, as illustrated in FIG. 1. Once UE 130 found a better cell served by the neighbour network node 120, it will send a measurement report to the serving network node 110. The serving network node 110 will then send a handover request to the neighbour network node 120 and if the neighbour network node 120 has enough resource to accept the handover, the neighbour network node 120 will send the handover acknowledgement to the serving network node 110. The serving network node 110 will send RRC Reconfiguration Request to UE 130 after forwarding the related data to the neighbour network node 120 and UE 130 will trigger the random access procedure to the neighbour network node 120.
In particular, UE 130 will send a random access preamble message to neighbour network node 120 (referred to as MSG1), neighbour network node 120 then sends the message indicating uplink grant to UE 130 (referred to as MSG2). UE 130 will send the UE related information and RRC Reconfiguration Complete message to the neighbour network node 120 based on the uplink grant in MSG2. That is, if the uplink grant is enough, UE 130 will send out both UE related information and RRC Reconfiguration Complete message through a single Message 3 (hereafter referred to as MSG3), and if the uplink grant is not enough, UE 130 will only send out the UE related information through the MSG3. In the later situation, RRC Reconfiguration Complete message will be sent on Message 5 (referred to as MSG5) to the neighbour network node 120 based on the uplink grant on Message 4 (referred to as MSG4) sent from the neighbour network node 120. Thus, there will be certain handover delay if the granted MSG3 size is not large enough to contain the RRC Reconfiguration Complete message.
On the other hand, the granted MSG3 size will also impact the cell coverage. The below table 1 and table 2 shows the two analysis with MSG3 sizes of 9 bytes (Table 1) and 19 bytes (Table 2) respectively.
TABLE 1Maximum Pass Loss for MSG3 size 9 BytesMsg 3PRACH f0UE Output power0.20 W0.20 WUE Output power23 dBm23 dBmThermal noise−174−174Noise factor RBS33Payload size9 ByteNAQuality requirement1.0%Bandwidth0.54 MHz1.08 MHzNoise power−113.7−110.7SINR used−7.6−10.2RBS Sensistivity−121.2 dBm−120.9dBmAntenna gain18.518.5Feeder loss33Jumper loss0.50.5ASC insertion loss00Body loss00Penetration loss2020Fading margin4.94.9Max pathloss134.3 dB134.0 dBunloaded
TABLE 2Maximum Pass Loss for MSG3 size 19 bytesMsg 3PRACH f0UE Output power0.20 W0.20 WUE Output power23 dBm23 dBmThermal noise−174−174Noise factor RBS33Payload size19 ByteNAQuality requirement1.0%Bandwdth0.54 MHz1.08 MHzNoise power−113.7−110.7SINR used−4.0−10.2RBS Sensistivity−117.6 dBm−120.9dBmAntenna gain18.518.5Feeder loss33Jumper loss0.50.5ASC insertion loss00Body loss00Penetration loss2020Fading margin4.94.9Max pathloss130.7 dB134.0 dBunloaded
It can be observed from the above analysis in tables 1 and 2, cell coverage (Maximum Pathloss) is −3.3 db less than PRACH0, which is the bottleneck of the cell coverage. That is, the large MSG3 size will reduce the cell coverage around 23% in some typical scenarios. Therefore, a problem addressed herein is how to keep the cell coverage while reducing the handover delay according to real situations.
In order to balance the coverage and the handover delay, in most cases the MSG3 size is configurable by the operators according to the real situation. For example, in rural area, small MSG3 size is configured for relatively large coverage of the cell, whereas in urban area, large MSG3 size is configured for improving the latency. The scenario of typical rural area is shown in FIG. 1(a), the scenario of typical urban area is shown in FIG. 1(b) and the inner ring areas A1 and A2 indicate the coverage with a large MSG3 size, the outer ring areas B1 and B2 indicate the coverage with a small MSG3 size. In rural area, as illustrated in FIG. 1(a), the handover area C is the overlap between the outer areas B1 and B2, and in urban area, as illustrated in FIG. 1(b), the handover area C is the overlap between the inner areas A1 and A2.
The configuring of the MSG3 size is based on the assumption that the cell is a regular shape according to the ideal cell planning, however due to some obstacles such as buildings, walls, etc., the cell cannot be the regular shape. Therefore, configuring the MSG3 size based on the cell areas as mentioned above does not work well in real situation.